1. Field of the Invention
The present invention relates to a ceramic tube for a high-intensity discharge lamp obtained by inserting a plug member into a hollow member and by subjecting the members to a firing treatment, and to a method of producing the ceramic tube for a high-intensity discharge lamp.
2. Description of the Related Art
In ceramic metal halide lamps, a pair of electrodes are inserted into a ceramic tube for a high-intensity discharge lamp (hereinafter referred to simply as the tube), and a metal halide is ionized by the electrodes to show discharge emission.
Various examples of such tubes have been disclosed, and include those produced by assembling a plurality of members, those produced integrally as a single member, and those produced by bonding two members. In FIG. 10, a tube 1 is shown as an example of such tubes. The tube 1 is produced by assembling a hollow member 3 and a plug member 4. The hollow member 3 has an insertion opening 2 at one end and a bottom at the other end, and the plug member 4 is inserted into the insertion opening 2.
In the tube 1, the hollow member 3 contains a substantially cylindrical body 5, a closure 6 for closing one end of the body 5, and a long first electrode insert 7 protruding from one surface of the closure 6. Meanwhile, the plug member 4 contains a disc-shaped portion 8 and a long second electrode insert 9 protruding from one surface thereof. The side wall of the disc-shaped portion 8 is bonded to the inner wall of the insertion opening 2.
The hollow member 3 and the plug member 4 having such structures are generally composed of a translucent alumina ceramic. In FIG. 10, a pair of electrodes (not shown) are inserted into insertion holes 10, 11.
As shown in FIG. 10, the first electrode insert 7 (the insertion hole 10) and the second electrode insert 9 (the insertion hole 11) face each other on a common axis. Therefore, the electrodes (not shown) are arranged facing each other in the insertion holes 10, 11.
As described in Japanese Laid-Open Patent Publication Nos. 2002-117807 and 10-125230, the tube 1 can be obtained by the steps of preparing the hollow member 3 and the plug member 4 separately by extrusion or press forming, firing and shrinking the plug member 4, inserting the disc-shaped portion 8 of the plug member 4 into the open end of the insertion opening 2 in the hollow member 3, and subjecting the hollow member 3 and the inserted plug member 4 to a firing treatment. In the firing treatment, the hollow member 3 is sintered and shrunk, whereby the inner wall of the insertion opening 2 and the side wall of the disc-shaped portion 8 are integrally bonded.
As is clear from FIG. 2 of Japanese Laid-Open Patent Publication No. 2002-117807 and FIG. 2 of Japanese Laid-Open Patent Publication No. 10-125230, the insertion opening 2 in the tube 1 generally has a constant diameter in the vicinity of the open end which the disc-shaped portion 8 is inserted to. In such a structure, the disc-shaped portion 8 is often bonded in the inclined state to the insertion opening 2 as shown in FIG. 10.
This inclination is caused due to the irregularity of positions in which the bonding between the hollow member 3 and the disc-shaped portion 8 is started in the step of sintering and gradually shrinking the hollow member 3. Thus, when in a certain point on the circumference, the bonding of the disc-shaped portion 8 is started not in a position in the vicinity of the inside of the insertion opening 2 but in a position in the vicinity of the inlet side of the insertion opening 2, the hollow member 3 is shrunk in the axis direction toward the bonded position in the vicinity of the inlet side until also the position in the vicinity of the inside is bonded by further shrinkage. When in another point on the circumference, the bonding of the disc-shaped portion 8 is started in a position in the vicinity of the inside of the insertion opening 2, the hollow member 3 is shrunk in the axis direction toward the bonded position in the vicinity of the inside of the insertion opening 2. In a case where such different bonding behaviors are simultaneously caused in different points on the circumference, the ends of the hollow member 3 and the disc-shaped portion 8 are bonded at approximately the same height in the point in which the bonding is started in the vicinity of the inlet side, while the end of the disc-shaped portion 8 protrudes from the hollow member 3 in the point in which the bonding is started in the vicinity of the inside, so that the disc-shaped portion 8 is inclined.
In this case, the axis of the second electrode insert 9 in the plug member 4 is inclined from the axis of the first electrode insert 7 in the hollow member 3. Thus, the electrodes inserted in the insertion holes 10, 11 are not arranged facing each other on a common axis. This may cause the breakage of the member in a process of inserting the electrodes to produce a lamp, and may increase the discharge instability of the lamp. Furthermore, the discharge arc position variation is disadvantageously increased due to the position instability of the electrodes, resulting in difficulty in light distribution control.
In the process of bonding the disc-shaped portion 8 in the plug member 4 to the insertion opening 2 in the hollow member 3, the bonding may be started in a position in the vicinity of the inlet side and in a position in the vicinity of the inside of the insertion opening 2, and then the intermediate position may be bonded thereafter. In this case, a closed space is formed between the bonded positions, air remains in the space, and a cavity is formed in the space in the bonding process, thereby resulting in the generation of a scattered light causing glare and the deterioration of a lamp life. Particularly, when the plug member 4 is prepared by a press forming method, since the disc-shaped portion 8 tends to have a smaller diameter in the intermediate position, this problem is often caused.
In addition, in the tube 1, the closure 6 of the hollow member 3 and the disc-shaped portion 8 of the plug member 4 have the same in-line transmittance lower than that of the body 5 of the hollow member 3, and a light is mainly emitted from the body 5. Therefore, for example, when the tube 1 is used in combination with a reflecting mirror to form a lamp for emitting a light in one direction, the light cannot be emitted in the axis direction of the reflecting mirror disadvantageously, resulting in difficulty in light distribution control.
In view of this disadvantage, Japanese Laid-Open Patent Publication No. 2006-093046 discloses a tube with an improved light distribution property. The tube is obtained by assembling three members, and wherein a portion corresponding to the body of the hollow member and a portion corresponding to the disc-shaped portion of the plug member (or the closure of the hollow member) have different transmittances. However, also this tube cannot emit a light in the axis direction.